One of the common petroleum refinery processes for the upgrading of light ends to high octane gasoline is the isoparaffin-olefin alkylation process in which an isoparaffin, usually isobutene is alkylated with a light olefin, usually propylene, butene or mixtures of the two, to produce a high octane liquid product in the gasoline boiling range. The alkylate product is considered a premium gasoline blending component due to its high octane, low RVP, low sulfur content and low distillation T90 point. Two major isoparaffin-olefin alkylation process have become widely accepted in the refining industry, the sulfuric acid process and the hydrofluoric acid (HF) process which, while fundamentally similar, possess different characteristics arising from the different abilities of the two acids to catalyze the alkylation reaction. These two processes are by now well established in the refining industry and each is recognized as having its own technical and economic advantages and problems.
Current worldwide gasoline demand, along with more stringent environmental limitations, are driving refineries to expand and consider new alkylation units. While the process economics may in many cases favor the HF process, economics alone may not always be the determining factor and sulfuric acid alkylation has retained a major portion of the alkylation capacity in the industry. Many of the new projects are likely to choose sulfuric acid as catalyst as a result of perceptions concerning the safety and environmental concerns about HF alkylation in spite of its excellent plant safety and environmental record. Solid acid catalyst technology, while attractive in principle, is, however, quite far from being sufficiently well established for widespread commercial acceptance, leaving sulfuric acid as a currently viable option. in spite of reactor reliability issues, especially with rotating equipment and internals.
There are two major variants of the sulfuric acid alkylation process which differ principally in the means used to remove the heat of the alkylation reaction. The DuPont™ Stratco™ process, the dominant process also known as the effluent refrigeration process, uses a liquid full reactor/acid settler system in which the heat of reaction is removed by an internal tube bundle, making the reactor resemble a shell-and-tube type heat exchanger although with an agitator which is used to secure good contact between the acid and the hydrocarbon reactants. Control of the pressure in the flash drum maintains the contents of the reactor in the liquid phase at by appropriate control, the temperature of the reactant mixture is kept at a desired value. The autorefrigerant process pioneered by Esso (now ExxonMobil), by contrast, uses a reactor in which the heat of reaction is removed by operation at a pressure at which a portion of the hydrocarbon charge boils. The reactor used in the autorefrigerant process conventionally comprises a single horizontal vessel divided into mixing chambers with one to two separate pressure zones. Plug flow is achieved by cascade operation with the refrigerant and sulfuric acid being admitted at one end of the contactor and the olefin being introduced progressively in the mixing chambers by means of spargers with vigorous mixing provided by driven impellers or, in certain cases, eductor type mixers. The reactant mixture and acid catalyst passes from chamber to chamber over weirs and through a pressure let down between the two pressure zones. Vaporization of the refrigerant removes the heat of reaction and very low temperatures can be achieved while operating at low pressure.
Considering the available options for sulfuric acid alkylation, the main technical disadvantages of the Stratco contactor can be summarized as the following: liquid phase operation requires higher pressure than autorefrigerated reactors and therefore it operates at a higher temperature which promotes secondary reactions; the hydrodynamics in the contactor consist of high velocity pumping of liquid by the single mixer required to create high liquid velocities over the heat exchanger tubes. As a result, the emulsion is subjected to very high energy dissipation locally in the region of the mixer blades and low energy dissipation everywhere else; the compressor energy requirement is between 15 and 20% higher than with autorefrigerated systems; the capacity of a contactor is typically 1200 to 2000 BPD (about 1900-3200 hl/day) of alkylate. Therefore, a typical unit will require between 5 and 8 reactors; and each reactor needs its own settler, pumps, and control system.
With the autorefrigerant process, most units require only one reactor and settler. However, there are some disadvantages: Vessels can get very large: 12 -15 ft (about 3.6-4.5 m.) diameter and over 100 ft (about 30 m.) in length; long residence times affect alkylate quality due to alkylate decomposition reactions; multiple mixing zones require multiple mixers, motors, and gearboxes which has an effect on maintenance and reliability control resources; the horizontal configuration is considered less efficient in terms of volume utilization; and the large sizes make it less competitive for units smaller than 10 kBD (about 15900 hl/Day) whereas many refineries will normally be satisfied by a capacity of 5-10 kBD (about 8000-16000 hl/day).
Given the continued viability of the autorefrigerant sulfuric acid alkylation process, it would be desirable to incorporate improvements which negate or offset at least some of the disadvantages mentioned above.
The present invention is concerned with improvements to the autorefrigerant alkylation process. Accordingly, in this specification, the term “alkylation” is used to refer to the isoparaffin-olefin alkylation process of the petroleum refining industry in which a light olefin (C2-C6, usually C3-C4) is used to alkylate a light isoparaffin (C4-C6, usually isobutane) to produce a liquid alkylation product which is predominantly in the gasoline boiling range. The autorefrigerant alkylation process, referred to as such in this specification is the alkylation process in which heat of the alkylation reaction is removed by vaporization of a reactant hydrocarbon refrigerant. Exemplary patents describing variants of the autorefrigeration alkylation process include U.S. Pat. No. 2,429,205 (Jenny); U.S. Pat. No. 2,768,987 (Hart); U.S. Pat. No. 2,903,344 (Rollman); U.S. Pat. No. 3,170,002 (Kelso); U.S. Pat. No. 2,852,581 (Stiles); U.S. Pat. No. 2,859,259 (Stiles) and U.S. Pat. No. 2,920,124 (Stiles).